<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<style>
    /* 媒体查询，就是根据设备宽度的不同，
       去显示对应的样式
     */
  div{
      width: 100px;
      height: 100px;
      border: 1px solid #333;
      background: yellow;
  }
  @media (min-width:992px) and (max-width:1200px){
      div
      {
        width: 200px;
        background: black;
      }
  }
  @media (min-width:768px) and (max-width:992px){
      div
      {
        width: 300px;
        background: pink;
      }
      }
  @media  (max-width:768px){
      div
      {
        width: 300px;
        background: red;
      }
      }
</style>
<body>
    <div>

    </div>
</body>
</html>